Object Expected!! how to solve?

anyone can help me find the problem of this code. i hv check several time and didn't found any error or wrong spelling!!

the color part is the line that exists error!!

<html>
<head>
<script language ="JavaScript">

function GradeLevel(){

var mark = parseInt(document.form1.txtMark.value);

if (mark > 69) && (mark < 101){
document.writeln("<h1>You get grade A</h1>");

} else{
if (mark > 59) && (mark < 70){
document.writeln("<h1>You get grade B</h1>");

} else{
if (mark > 49) && (mark < 60){
document.writeln("<h1>You get grade C</h1>");

} else{
if (mark > 39) && (mark < 50){
document.writeln("<h1>You get grade D</h1>");

} else{
if (mark < 40) && (mark >= 0){
document.writeln("<h1>You get grade E</h1>");

} else{
document.writeln("<h1>Please enter the correct mark</h1>");
}
}
}
}
}
}

</script>

<title>untitle</title>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
</head>

<body>
<form name = "form1" method = "post" action ="">
<table width="444" height="52" border="1">
<tr>
<td width="164"><div align="center">Mark</div></td>
<td width="264"><input name="txtMark" type="text" id="txtMark"></td>
</tr>
<tr>
<td><div align="center">
<input name="cmdResult" type="button" id="cmdResult" value="Result" OnClick="GradeLevel()">
</div></td>
<td><input name="cmdClear" type="Reset" id="cmdClear" value="Clear"></td>
</tr>
</table>
</form>
</body>
</html>